Transparent electrical conductors are critical components in many electronic devices, including light emitting diodes, photovoltaics, flat-panel displays, and electrochromic devices. Consequently, there is a growing demand for low-cost transparent conductors that offer not only high conductivity and transparency over a large wavelength range, but an array of other properties such as good mechanical flexibility, environmental stability, and desirable surface morphology. Currently, the most widely used transparent conductor for electronic devices is indium tin oxide (ITO). However, ITO is hindered by its relative brittleness, which degrades its performance on flexible substrates, and the limited availability of indium, a rare and expensive element obtained as a by-product of mining for other elements.
Carbon nanotubes recently have emerged as a promising alternative to ITO for transparent electrical conduction. These nanomaterials consist entirely of carbon, one of the most abundant elements on earth, and exhibit both remarkably high conductivities and exceptional mechanical properties including high tensile strength and resilience. Single-walled carbon nanotubes (SWNTs) can be thought of as nanoscale tubes formed by rolling a graphene sheet into a seamless cylinder. As a result of this structure, SWNTs are available in a large number of different chiralities—combinations of diameter and wrapping angle. The nanotube chirality defines both its electronic and optical properties, and hence is a critical parameter when incorporating nanotubes into device applications. For instance, roughly two thirds of SWNT chiralities are semiconducting, while the rest are metallic. Moreover, the first-order peaks in optical absorbance for metallic SWNTs can vary widely from roughly 450 nm to 700 nm as SWNT diameter is increased from ˜0.7 nm to 1.4 nm. Although this striking dependence between SWNT atomic structure and behavior enables them to be employed in many ways, it is also regarded as one of their major weaknesses as there exist no methods of synthesizing SWNTs of uniform chirality. Instead, as-synthesized SWNTs possess a mixture of semiconducting and metallic nanotubes with varying diameters.
Transparent, electrically conductive films of carbon nanotubes have been fabricated from solvent suspensions using a number of different methods, such as airbrushing (see e.g., U.S. Patent Application Publication No. US 2005/0221016; and M. Kaempgen et al., Appl. Surf. Sci. 252, 425 (2005)), drop-drying (see e.g., U.S. Pat. No. 5,853,877), and vacuum filtration (see e.g., U.S. Patent Application Publication No. 2004/0197546; Z. Wu et al, Science 305, 1273 (2004); and Y. Zhou et al., Appl. Phys. Lett. 88, 123109 (2006)), the entire disclosure of each of which is incorporated by reference herein. Prior art, however, has employed unsorted mixtures with roughly 2:1 ratios of semiconducting and metallic carbon nanotubes which limit device performance since two thirds of the SWNTs are semiconducting and thus possess inferior electrical conductivity. To increase film conductivity, SWNTs can be chemically doped in strong oxidizing conditions such as through nitric acid refluxing (see e.g., A. G. Rinzler et al., Appl. Phys. A, 67, 29-37 (1998)). These treatments, however, can introduce defects into the nanotubes and reduce their length. Furthermore, these treatments lead to decreased film transmittance in the infrared portion of the electromagnetic spectrum. Already prepared films also can be doped through immersion in agents such as nitric acid, sulfuric acid, and thionyl chloride (see e.g., R. Graupner et al., Phys. Chem. Chem. Phys. 5, 5472 (2003); and D. Zhang et al., Nano Lett. 6, 1880 (2006)), or exposure to elements such as halogens or alkali metals see U.S. Patent Application Publication No. 2004/0197546). However, such treatments rely on intercalation and adsorption of molecules, and generally can be reversed by rinsing in water. Prior art also has employed several techniques to achieve beneficial nanotube-nanotube contacts for improved transparent conduction. For example, water rinsing and bath sonication have been attempted to induce SWNT rebundling in already prepared nanotube films (see U.S. Patent Application Publication No. 2005/0221016). Hecht et al. have studied the dimensions of SWNT bundles as a function of sonication time with the aim of improving the performance of transparent conductors (see D. Hecht et al., Appl. Phys. Lett. 89, 133112 (2006)).